452 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
This species, so far as these specimens indicate its character, appears to 
resemble the genus Stenegjiber, from the lower Phocene formation of Saint- 
Gérand-le-Puy, France, both in the general form of the skull and in its den- 
tition. So great is this resemblance that Dr. Leidy at first referred it to 
that genus, which Kaup has regarded as identical with Chalicomys. The lower 
jaw in Steneofiber, says Leidy, is unknown, and adds that that of Paleocastor 
is quite unlike the lower jaw in Chalicomys. The structure of the molars in 
Chalicomys differs greatly, as shown by Gervais’s figures,* from that of Steneo- 
fiber, and Geoffroy, Gervais, and others regard the two forms as generically 
distinct. In Paleocastor, the structure of the molars is very similar to what 
is scen in the figures of the molars of Steneofiber. In respect to the skull, 
Leidy also observes that. the ‘‘forehead presents the same triangular form and 
proportionate size” as in Steneofiber. “The temporal fossze”, he continues, 
“appear to have had the same form and proportional capacity. ‘They were 
separated in the same manner by a long sagittal crest, extending forward upon 
the frontal bone.” The cranium, just back of the forehead, was equally con- 
stricted. The external auditory passage formed a short, oblique canal, with 
its orifice directed outward and backward in the same manner. The palatal 
region, likewise, had the same form and construction, and the infra-orbital for- 
amen held the same relative position as in Steneofiber viciacensis. The inci- 
sors in both jaws are proportionately as long and strong as in the Beaver, 
and they have the same form.” Dr. Leidy adds that they also strongly 
approach in form those of the Beaver. 
The molar teeth in “ Palaocastor” differ very greatly in structure from 
those of either Castor, Eucastor, or Trogontherium, more resembling, appar- 
ently, as do also those of Steneofiber viciacensis, those of some members of 
the Dasyproctida. 'The skull also differs greatly in form from that seen in 
these genera in consequence of the much greater constriction above of the 
interorbital region. Hence ‘ Palzocastor”, if really belonging to the family 
Castorid@, probably differed very much in general structure from the existing 
Beavers, and has its nearest ally in the genus Sveneofibert of the Miocene 
epoch of Europe. 
* Zool. et paléont. frang., plates i, viii, and xlviii. 
t Gervais says, “Les genres Chalicomys, Kaup, et Steneofiber, E. Geoff., sont é6videmment de la méme 
tribu que les Castors” (Zool. et paléont. frang., p. 20), to which group they have been generally referred 
by subsequent writers. Mr. E. R. Alston, however, in his recent memoir “On the Classification of the 
Order Glires” (P. Z. S., 1876, pp. 61-98), gives both Chalicomys of Kaup and Pa/wocastor of Leidy among 
the “more doubtful fossil genera” of the family Castoride, as well as the genera Palwomys and Chelodus 
of Kaup, and the genus Trogontherium of Fischer (not Trogontherium of Owen, = Diobroticus, Pomel). 
